Pascale Chabanet, ECOMAR , La Réunion 

and Mireille Harmelin-Vivien, DIMAR, COM

The coral reefs are the marine ecosystem with the greatest variety of species. They are comparable in this respect to the primary tropical forests as far as the terrestrial ecosystems are concerned.

The fish inhabiting the coral reefs have often been compared to the birds inhabiting the tropical forests because of the variety of their populations and the structural complexity of their habitats. This great diversity has given rise to a highly complex set of relationships between the various taxinomic groups cohabiting the coral ecosystems.

Fish occupy the whole living space in and around the reef itself. They show a clear-cut pattern of vertical distribution, depending on the type of habitat colonised and on the animals’ deep sea habits.

Three main ecological categories have been defined as follows:

• Species living on or near the sediment,

• Species living in close contact with the coral complexes,

• Species swimming out in the open seas.

Some examples of the 3 main categories of fish :

The vertical pattern of distribution of the fish obeys a dynamic process which varies with time, especially according to a nycthemeral rhythm. Most of the fish move from one compartment to another in the evening and back again in the morning.

The fish also show a horizontal pattern of distribution depending on the various biotopes or geomorphological zones they encounter on the coral reefs. Inside the fringing reefs, for example, the distinction can be made between the fish populations living at the back of the reef, those inhabiting the inner reef flats and those inhabiting the outer flats.

These fish populations are all the more clearly distinguishable when the reef itself is more highly structured, and they tend to overlap when the morphological components of the reef are relatively uniform.

Reproduction and development of the fish:

The vast majority of the fish inhabiting these regions are oviparous and lay their eggs in the sea. The eggs develop into larvae, which undergo several stages of pelagic development. The larvae drift with the currents and feed at first on phytoplankton, and then on zooplankton as they grow older. Some of them are able to swim actively, guided by various environmental factors. When they reach the juvenile stage, they recolonise the lagoons after crossing the ridge of the reef; they continue to move around until they have detected the most suitable place in which to settle. This phase during which the reef habitat is colonised is called the recruitment phase. Some larvae settle down and become juveniles only a few days after their eclosion, while others have a much longer larval stage lasting up to more than 2 months. Once they have found a suitable place in which to settle, the larvae move to the bottom, where they soon acquire the characteristics of juveniles. Some of the reef species are  hermaphrodites and make a change of sex during their lifetime (successive hermaphrodism).

Feeding behaviour:

The diet of the various species depends mainly on the size of the individuals, the time of year and the  biotope involved.

Most of the fish living on and around the reefs have a specific feeding rhythm. Some of them are strictly diurnal in their feeding habits, while others are mainly nocturnal. Some show a peak in their feeding activity during the twilight periods and others’ eating habits do  not seem to depend on the light. The differences between the patterns of eating activity make for distinct populations, some of which are active during the day and others at night. They succeed each other according to a nycthemera pattern. The populations change places during the twilight periods in a specific order depending on the light intensity:

• the strictly diurnal species hide during the night in the hollows of fhe coral complexes or buried in the sediment. This group includes all the herbivores, all the omnivores, the sessile invertebrate grazers, the plankton-eaters and some carnivores;

• the nocturnal species are all carnivores.

75 % of the ichtyological species inhabiting the reefs are active during the day.

Three main classes have been defined in terms of their diet, as follows:

The fish inhabiting the coral reefs belong to 2 classes :

The orders and families most frequently observed among the Osteichthyes